This invention relates to a method of coding a digital video signal representative of successive pictures into a coded signal by the use of correlation between two successive ones of the pictures. This invention relates also to a coding device for use in carrying out the method.
Each of the successive pictures may correspond to a frame of the digital video signal when the digital video signal is, for example, a digital television signal. On transmitting the digital video signal as transmission data, it is known in the art that an interframe predictive coding method is effective to achieve data compression of the transmission data and to thereby reduce the amount of the transmission data to be transmitted. The interframe predictive coding method makes use of correlation between two successive ones of successive frames. More particularly, a difference is at first calculated between corresponding picture elements of two successive pictures. Such differences are coded into a coded signal which represents the transmission data and is received as reproduced pictures. The difference will be called an interframe difference.
The interframe predictive coding method is, however, defective in that the reproduced pictures become to have a poor quality when the high compression is retained even for pictures which include a movement. This is because the difference becomes large as a result of a decrease in the correlation between the two successive pictures.
In order to remove the defect, an improved method is proposed which is called a motion-compensated interframe coding method. In the improved method, each of the successive pictures is divided into a predetermined number of blocks of picture elements. As will later be described more in detail, a motion vector is detected which represents a movement of each block between two successive pictures. By using the motion vector, prediction is carried out with a high accuracy. This renders the difference small between the picture elements of the two successive pictures which include the movement. The improved method is disclosed, for example, by Tatuso ISHIGURO and Kazumoto IINUMA in an IEEE Communications Magazine, November 1982, pages 24-30, under the title of "Television Bandwidth Compression Transmission by Motion-compensated Interframe Coding."
However, the improved method is still defective in that the amount of the transmission data becomes large when a drastical or large change, such as a scene change, occurs between the two successive pictures. This is because the correlation between two successive pictures is possessed of an extremely lower degree of correlation when the drastical change occurs.
The amount of transmission data could be reduced if the high compression were retained even when the drastical change occurs. In order to reduce the amount of the transmission data, the digital video signal should be subjected to a rough coding method in the manner which will presently be described. On applying a rough coding method to the digital video signal, it is possible to use a rough quantization step on quantizing the digital video signal or to delete either a field or a frame of the digital video signal. It is also possible for a worst case to suspend coding the digital video signal at an intermediate part of the picture. When the rough quantization step or deletion of the field or the frame is carried out, the reproduced pictures inevitably have a low quality. When coding the digital video signal is suspended, freezing may occur in the reproduced picture. More specifically, each of such reproduced pictures may have an upper already coded area in combination with a remaining area which is not yet coded. The upper area corresponds to an upper portion of a new picture. The remaining area corresponds to a lower area of a previous picture. The freezing continues a predetermined time duration of, for example, 0.3 seconds. Inasmuch as the freezing continues an appreciably long time duration, the freezing is eyesore to viewers. Thus, suspension of coding the digital video signal also results in a reduction of the quality of the reproduced picture.